The hypothesis for this study is that intake of 20% glycerol solution at regular intervals will reduce the volume of fluid loss during headdown bedrest. On one headdown session subjects will during 20% glycerol at specific times and on another session total fluid intake will be the same with no glycerol. Monitoring of session total fluid intake will be the same with no glycerol. Monitoring if urine volume, plasma osmodality, urinary aldosterone, hematocrit and hemoglobin will be the major data collection.